The present invention relates generally to PTFE dispersion polymerization techniques, and particularly to a new class of rate enhancing additives for use in PTFE dispersion polymerization.
Polytetrafluoroethylene (PTFE) homopolymer and copolymers of tetrafluoroethylene (TFE) and ethylenically unsaturated comonomers can be produced in aqueous media by two distinctly different procedures. In one procedure, little or no dispersing additives, or surfactants, are added, and vigorous agitation is provided, yielding a precipitated "granular" resin. In the second procedure, a dispersing additive or surfactant is added, and mild agitation provided, to yield a colloidal suspension of small polymer particulates. The products of the two processes are employed in different applications; granular resin is fabricated into products by molding, while the resin produced by dispersion polymerization is employed in dispersion coating and paste extrusion fabrication processes.
Numerous classes of suitable surfactants for PTFE dispersion polymerization have been proposed and evaluated. Included within these classes are polyfluoroalkanoic acids and salts thereof; polyfluorosulfonic acids and salts thereof; polyfluorophosphonic acids and salts thereof; sulfuric and phosphoric esters of polyfluoroalkanols; and polyfluoroalkylamine salts. These classes of surfactants are disclosed by Berry, U.S. Pat. No. 2,559,752, among others. Other useful general references relating to PTFE dispersion polymerization processes include Punderson, U.S. Pat. No. 3,391,099; Porter, U.S. Pat. No. 3,704,285; Gangal, U.S. Pat. No. 4,186,121; and Khan, U.S. Pat. No. 4,380,681.
Of the foregoing classes of surfactants, the most widely employed are sodium and ammonium salts of perfluoromonocarboxylic acids having 6 to 10 carbon atoms, especially ammonium perfluorooctanoate.
The foregoing classes of surfactants can be structurally characterized as bifunctional compounds having regions of different polarity; specifically, each surfactant molecule comprises a hydrophobic group and a hydrophilic group. In dispersion polymerization processes, the hydrophobic group is adsorbed onto the surface of a growing particle, while the hydrophilic group remains in the aqueous phase. The hydrophilic group is hydrated in the aqueous phase and in the case of ionic surfactants, it is also ionized, imparting a charge to the particle surface. As a result of their like-charged surfaces, individual particles repel each other, preventing coagulation. When conventional surfactants are employed, the aqueous surface tension of the aqueous phase is initially low. The rate of polymerization is initially fast, but declines or moderates as surfactant is absorbed onto the surface of the increasing population of PTFE particles. This adsorption process stabilizes the polymer dispersion but results in increased surface tension relative to initial conditions, and, consequently, a reduced flux of TFE from the gas phase to the aqueous phase.
It has now been found that certain low molecular weight tertiary perfluorinated compounds are effective rate enhancing additives for use in TFE dispersion polymerization. These compounds act to reduce the surface tension of the aqueous polymerization medium throughout the polymerization process, enhancing flux of TFE gas from the gas phase to the aqueous phase. These rate enhancing agents remain in the aqueous phase throughout the polymerization process. Heretofore, increases in polymerization rates, and hence, process productivity, could be obtained only by increasing concentrations of polymerization initiator. However, this expedient provides polymer having reduced average molecular weight. Use of the rate enhancing compounds of the present invention provides significantly increased polymerization rates without adversely affecting the size of polymer chains or the morphology of the resulting dispersed polymer particles.